The present invention relates to a circuit arrangement for starting and operating at least one discharge lamp with a first input terminal and a second input terminal for connecting a supply voltage, an inverter, which includes at least one first main transistor and one second main transistor in a half-bridge arrangement, which main transistors are coupled in series between the first input terminal and the second input terminal, a first output terminal and a second output terminal for connecting the at least one discharge lamp, at least one lamp inductor, which is coupled in series with the first output terminal, at least one capacitor, which is coupled in parallel with the first output terminal and the second output terminal, a transformer with a primary winding and a first secondary winding and a second secondary winding, a series circuit including the primary winding and the at least one lamp inductor being coupled between the half-bridge center point and a reference potential, and a first control circuit for driving the first main transistor and a second control circuit for driving the second main transistor, each control circuit having an input and an output, the output of the first control circuit being coupled to the control electrode of the first main transistor, and the output of the second control circuit being coupled to the control electrode of the second main transistor, with the input of the first control circuit being coupled to the first secondary winding and the input of the second control circuit being coupled to the second secondary winding, each control circuit having a timing circuit, whose time constant varies as a function of the voltage across the input of the respective control circuit, each timing circuit having at least one first auxiliary transistor, the working electrode of the first auxiliary transistor being coupled to the control electrode of the associated main transistor and the reference electrode of the first auxiliary transistor being coupled to a reference potential, the control electrode of the first auxiliary transistor being coupled to the center point of a frequency-dependent voltage divider, which is coupled firstly to the respective secondary winding and secondly to the respective reference potential.
Such a circuit arrangement is known from EP 0 093 469 A1. In this case, the frequency-dependent voltage divider of each timing circuit includes a nonreactive resistor and a capacitor, the voltage drop across the capacitor being coupled to the control path of the first auxiliary transistor. Furthermore, zener diodes are provided which are used as relatively precise thresholds for the generation of the open-circuit voltage for the discharge lamp, i.e. the voltage which is available across the lamp for starting.
One disadvantage with the solution in accordance with the mentioned document is the considerable control losses, since precise zener diodes with a low dynamic resistance, as are required for this solution, are only available above approximately 7 V, and the voltage generated via a current-measuring resistor during lamp operation is of this order of magnitude. More detailed consideration of the mentioned solution shows that a reduction in the value of the nonreactive resistor of the frequency-dependent voltage divider needs to be compensated for by enlarging the capacitor of the frequency-dependent voltage divider. Since the dynamic resistance of a zener diode increases with a lower zener diode voltage, as would be required when decreasing the voltage across the secondary winding in order to reduce the control losses, this results in a higher time constant. The delay of the voltage across the control electrode of the first auxiliary transistor in comparison with the current into the control electrode of the main transistor is thus undefined. This also results in a large, undesired spread. A reduction in the voltage drop across the respective secondary winding in order to decrease the control losses thus results in an increase in the tolerances.
The current-limiting effect of the lamp inductor is reduced as a result of saturation which may occur, as a result of which the main transistors of the inverter can be destroyed. It should be taken into consideration here that operation of the lamp inductor with slight saturation is desirable, however, since this results in low losses. It is precisely at this point that an increase in the tolerances becomes noticeable as a negative factor since, as a result, the risk of saturation of the lamp inductor arises. As a result, a reduction in the control losses would in this way result in the component parts of the circuit arrangement being provided with increased dimensions and therefore in increased costs.